34 J. P. HILL. 



equally evident from an inspection of the micro-photog-raplis, 

 figs. 24-20. In the larg-er cells of the lower ring- (fig. 24, 

 tr.ect.) the nucleus (rich in chromatin and nucleolated) 

 is surrounded by a perinuclear zone of clearer, coarsely 

 vacuolar cytoplasm, outside of which is a densely granular 

 deutoplasmic zone, which extends to within a short distance 

 of the periphery of the cell-body. In the smaller cells of the 

 upper ring (fig. 25, /.c.) the cytoplasm is coai-sely reticulai-, 

 with a tendency to compactness round the nucleus, and its con- 

 tained deutoplasmic material is spare in amount as compared 

 with that of the lower cells, being mainly located in a quite 

 narrow peripheral zone. The upper cells thus appear relatively 

 clear as compared with the dense, opaque-looking lower cells 

 (fig. 26). 



It becomes evident, then, that we have to do here, in this 

 fourth cleavage generation, with an unequal qualitative 

 division of the cytoplasm of the blastomeres of the 8-celled 

 stage. Just such a division as this we should expect if the 

 deutoplasmic material were mainly aggregated towards the 

 lower poles of the dividing cells. The evidence shows that 

 this is actually the case. In the 2-celled and especially in the 

 4-celled eggs we have already seen that the deutoplasmic 

 network is already most strongly developed towards the lower 

 poles of the blastomeres. This polar concentration of the 

 deutoplasm reaches its maximum in blastomeres of the 8- 

 celled stage, and confers on these an obvious polarity. 

 Although I failed to obtain normal examples of the latter stage, 

 I have fortunately been able to observe the characters of the 

 blastomeres in sections of eggs with twelve, thirteen, and 

 fourteen cells respectively. 



In the 12-cellcd egg (PI. G, fig. 57), measuring '38 mm. 

 in diameter, four of the eight original blastomeres are still 

 undivided ; the remaining four have undergone division 

 unequally and qualitatively, one but recently, so that 4 + 

 (4x2) = 12. The undivided blastomeres are large (average 

 diameter, •]! x '070 mm.) and ovoidal in form, their lower 

 ends being thicker than their upper, and they exhibit a well- 



